Abrasive article and its manufacture



NOV. 15, J. A. BOYEF? ABRASIVE ARTICLE AND ITS IVLMIUFACTURE Filed June 28, 1937 0 "7o SILVER 72% 100 0% 57 7 01% Cal INVENTOR. JOHN A. BO ER ATTORNEY.

Patented Nov. 15, 1938 John A. Boyer, Niagai-a Falls, N. Y., assignor to The Carborundum Company, Niagara Falls, N. Y a. corporation of Delaware Application June 28, 1937, Serial No. 150,766

3 Claims. (Cl. 5l280) This invention relates to metal bonded abrasive articles and their manufacture, and particularly to the metal bonding of such abrasives as silicon carbide, boron carbide, fused alumina 5 and diamonds.

One of the objects of the invention is to produce a metal bonded abrasive article having improved grinding or lapping characteristics. Another object of the invention is to produce a metal bonded abrasive article in which the bond or matrix is characterized by high degree of coherence and strength, and in. which the bonding material can be matured at a relatively low temperature. A further object is to produce 5 a metal bond or matrix for abrasives in which the coalescence of the metal particles is facilitated by comparative freedom of the metal from surface films of oxide. A further object is to provide a metal bond or matrix which will satis- 20 factorily retain the abrasive during lapping or grinding, but which can be readily dressed so as to present a free cutting surface. These and other objects will be apparent from the present specification.

In the bonding of abrasives with metal, itis difiicult to provide a metal or alloy which has the characteristics required for a satisfactory abrasive bond. The non-metallic materials used for bonding abrasives are brittle, and efforts in 30 connection withv metal bonding have been directed principally towards duplicating this property by the use of hard or brittle materials. When the bond or matrix is extremely hard, it is difiicult to dress the wheel after the abrasive 35 becomes worn, so that a fresh cutting surface cannot readily be presented. When the bond or matrix is brittle, it chipsout upon grinding with consequent loss of abrasive. A brittle matrix has the further disadvantage that when the 40 abrasive contentis increased so as to become a major portion of the wheel, the bond or matrix is weakened by the discontinuous structure caused by the presence of the abrasive particles. In metal bonded wheels having a high abrasive 45 concentration or in wheels in which extremely fine grits are employed, it is, necessary to provide a strong metallic structure throughout the wheel ii any advantage is to be derived from .the use of metal as a bonding agent.

50 In the production of metal bonded abrasives, I have found that silver and silver alloys possess very desirable bonding characteristics. Silver has no amnity for oxygen at high temperatures,

and as the particles of silver are entirely ,free

55 fromoxide films, the powder sinters readily into a coherent mass and at the same time retains the abrasive so as to produce a wheelhaving a high cutting emciency. Other metals difluse rapidly into silver when the metal is heated to so temperatures substantially below its melting point, so that sintered alloys can be readily .formed from a mixture of the component metal wheels I have been able to out very thin sections of quartz for radio frequency apparatus where even the slightest degree of localized heating will crack the section being out.

For the cutting of glass and other materials which have a dressing actionon the wheel, it is possible to use avery duc 'le matrix such as pure silver, providing suficien abrasive is present in the wheel to prevent rapid wearing away of the metal matrix.

In abrasive articles in which diamonds are bonded with silver or silver alloys, the wear resistance of the matrix can be substantially increased by the addition of particles of other abrasives such as boron carbide, silicon carbide or fused alumina. This additional abrasive material, even though it may do practlcally'no cutting in comparison with the cutting done by the diamonds, stilfens the metal matrix and makes it more resistant to wear and abrasion. diamond content can thus be reduced to a relativelylow percentage and the wheel will still have a satisfactory cutting rate and a long life.

Although puresilver forms a satisfactory bond for many purposes, it is usually desirable to harden the silver by the addition of alloying agents. Silver has the property of retaining a number of metals in solid solution, i. e., in hot mogeneous' admixture therewith in thesolid state, up to fairly high percentages, and the resulting alloys are hard, strong and ductile. Another group of alloys in which only a part of the alloying ingredients are retained in solid solution, namely, alloys of silver, copper and zinc or cadmium, which will be further described, have a strength approaching that of mild steel, the ductility of brass or-bronze, the resistance to oxidation of a heat-resistant alloy and melting points as low as 700 to 750 C. I know of no nation of properties. When alloys of this type are used for the bondlng'of the abrasive particles, they satisfactorily retain the abrasive and provide a. metal matrix having a. very high degree of coherence and strength. The low sintering or melting temperatures are of particular advantage in the commercial production of Theother metallic materials possessing this combiill (all

bonded abrasives, especially in the case of combustilole or readily oxidizahle abrasive materials silver-cadmimn alloys from il to 61% cadmium,

which is typical of the equilibrium relations of alloys in which silver retains a high percentage of another metal in solid solution;

Figure 3 shows a plan View of an abrasive wheel in which the periphery contains diamonds and. the inner portion of the Wheel is composed of metal'ancl an abrasive other than diamonds;

Figure 4 shows a section of a wheel which is adapted for the facing of tungsten carbide tool points by sirle grinding; and

Figure 5 shows a plan View of the wheel illustrated in Yigtue The efieci; of the copper upon the melting tern goeratures and the sintering temperatures of the silver-copper alloys may he understood from the equilibrium of this alloy series, which is shownin Figure 1.

although copper has a higher melting point than silver, it reduces the melting point or" the silver when added to it as an alloying ingredient. The copper also has a moderate hardening effect, and produces a bond which is somewhat harder than pure silver and at the same time is free iron: brittleness. complete melting of the silver copper alloys is renresented by the curve 2 and. the temperature,

oi incipien melting is shown by the line For practically all alloys of the series this incipient melting point is 778. (3., as compared with 9532" C. for the melting point of pure silver. Most of the alloys melt over a range of temperatures but the alloy containing 72% silver has a constant melting point of 778 C.

in making a metal bonded abrasive from a silver-cop oer alloy containing a high percentage of silver, it is desirable to use the eutectic com-- position containing 28% copper. The metals can be previously alloyed and then comminuted to powder, and upon pressing and heating, the bond will soften and coalesce at the temperature below 278 C. If it is desired to completely melt the coed, melting will talre place when a tem perature of 773" C. is reached or exceeded, as is indicated by the point i on the diagram shown in Figure l.

An alloy bond or matrix of the eutectic corhposition can also be prepared by mixing the component metals in powdered form with the abrasire and subsequently applying heat and pressure. The articles can be pressed during the heating process or they can be cold molded and subsequently heated to a temperature slightly below 7V8 C. If a molten bond is desired, the temperautre of 773 C. can be exceeded.

Silver can retain zinc and cadmium in solid solution up to fairly high percentages, and these solid solutions provide bonds which are harder than pure silver but at the same time are ductile.

figure the temperatures of incipient melting and of complete melting are represented by the curves t and l resmectively. All compositions up.

to about s*r% cadmium are ductile solid solutions the diagram, the temperature of of the type ordinarily designated in metallurgical practice as alpha solid solutions. In solid solutions of this type, a pure metal forms the basis for the lattice structure as'determlned by X-ray difiraction, and the addition of the second metal merely produces a change in the lattice dimensions without producing an undissolved constituent. After the composition of 37% cad iniuiii is reached, a second metallic constituent appears which is harder than the solid solution and which can act as an additional hardening agent. This material is, however, brittle and the brittleness oi the alloy increases as the cadmium content is increased. 7

Silver can also retain zinc in solid solution up to about 25%, and the alpha solid solution is characterized by properties similar to or the alpha solid solution of cadmium. The ternary alloys or silver, and earls ii"- to have the general properties. previously pointed ou t possible to exceed the solid solubility limit in these alloys and obtain a further hardening effect by the production of a second constituent without producing extreme brittleness. The adcli of copper in amounts of, for example, from 5 to 10% to these alloys also produces further hardening. With higher additions of copper, the alloys become relatively low melting, and come Within general classification of silver: solder.

The following are examples o: liver solders which form suitable abrasive bonds or matrices. alloys are particularly adapted for the bonding of diamonds because their extremely low melting points combined with strength. to the relatively high silver content of the alloys they can lee sinterecl iii-to a coherent mass from powder at temperatures which have no injurious efiect upon diamond.

Silver Copper Zinc Cadmium phorus so 5 75 2o 5 we so 65 2c 15 85 2o 9 6 65 2c 15 so i5 till l5 l5 l5 5 so is 15 2o ill 36 25 ill 5 Silicon can also he added to pure silver or to silver alloys to produce a hardening effect upon the abrasive bond. When silicon is added to pure silver it does not form an intermetallic compound, and the resulting alloy consists of silicon particles embedded in a continuous silver matrix. The juncture between the surface oi" the silicon particles and the silver matrix is, however, a true alloy joint. Since the silver matrix: is practically continuous a substantial hardening eiilect can be obtained without causing a high degree of brittleness. Compositions of, for example, from 5 to 463% silicon can be used. When silicon is added to silver alloys containing copper or other metals which form sillcldes, the resulting compound formed with the silicon, although hard, has a tendency to lee brittle, and the properties of the send can be controlled over fairly wide limits by the introduction of these brittle compounds of silicon'in various proportions.

The alloys of silver and copper with phosphones are low melting, and have properties similar to those of the silver solders containing zinc or cadmium.

In bonding silicon carbide or fused alumina with silver alloys, or in the bonding of diamond or boron carbide for purposes where an extremely high cutting rate is necessary, it may be desirable to employ an alloy having brittle characteristics. Such alloys can be produced from silver by appreciably exceeding the solid solubility limits (as for example in the case of zinc and cadmium), by the production of brittle intermetallic compounds such as the silicides by the addition of silicon in fairly high percentages, or by the addition of arsenic and antimony which can be added in minor proportions either to silver or to the.

above described alloys. Silver sulphide is also soluble in silver, and an alloy of silver and silver sulphide will produce an embrittling effect when it is desired to obtain a brittle bond.

The addition of tin to silver in minor percentages (as for example from 5 to 20%) also are not intended to be limiting in regard to alloy compositions, grit sizes or methods of molding and sintering.

' Escample I A peripheral grinding wheel of the type shown in Figure 3 can be made as follows:

A cylindrical mold is separated into an inner and an outer compartment with a paper separator, the width of the outer compartment being that desired for the rim 9 containing'the cutting abrasive. Into this outer portion of the mold is placed a mixture of 7 per cent diamonds, 10 per cent silicon carbide, 25 per cent copper powder, and 58 per cent silver powder. The diamonds can be from 100 to'200 grit; the silicfin carbide from 130 to 600 grit, and the metal powder finer than 200 mesh, The inner space within the mold is filled with a mixture of 20 per cent silicon carbide, 24 per cent copper powder and 56 per cent silver powder, using approximately the same particle sizes as in the outer rim. The mixes in the respective compartments are levelled off, the paper separator removed, the mix again levelled, and subjected to pressure of, for example, from 10,000 to 50,000 lbs. per square inch. The pressed wheel is then removed from the mold, and is sintered in a non-oxidizing atmosphere at a temperature of 700 to 750 C. The wheel may be placed between two ceramic bats and sintered under a slight pressure to prevent warping.

Example II A wheel for the side grinding of tungsten oarbide tools, such as is illustrated in Figures 4 and 5, can be made as follows:

A composite abrasive ring is molded by first placing in a ring-shaped mold a mixture of 10 per cent diamonds of 100 to 200 grit, 10 per cent silicon carbide of from 180 to 600 grit, 52 per cent silver powder, 16 per cent copper powder, and 12 percent zinc powder. This mixture is levelled thin, it is advisable in molding the ring l2 to off, r and a, backing mixture composed of 75 per cent copper powder and 25 per cent of silicon,

carbide of a grit size corresponding approximately to the grits used in the abrasive layer. The back ing layer is levelled off, and the composite ring Example III A previously alloyed silver solder containing 60 per cent silver, 25 per cent copper and 15 per cent zinc is comminuted to powder. der is mixed with abrasive grain in the desired proportions, as for example, with from 5 to 20 per cent diamonds, with diamonds and an abrasive of a lesser degree of hardness as described in the two previous examples, with boron carbide mixed The metal powwith silicon carbide or fused alumina, or with boron carbide, silicon carbide or fused alumina alone. In using silicon carbide or fused alumina, the abrasive content can be increased to fairly high percentages if desired. The article can then be molded and sintered by any suitable method, as for example, by the methods described in detail in the previous examples. As an alternative to the molding of the material while cold under a high pressure, the abrasive mix and the mix for the metal backing can be introduced into a carbon mold and the composite ring sintered under pressure. Itis possible by this method to sinter an abrasive mix directly ontoa solid metal backing.

In the wheelshown in Figure 3, only the outer rim or periphery 9 is used for grinding. It is de sirable however, to add a quantity of abrasive or other inert filler material in the inner portion ill of the wheel so as' to make the shrinkageof the inner part of the wheel comparable withthe outer ring which is intended to do thecutti'ng. This,

,for the cutting of such materials as tungsten carbide tools, glass or silicon carbide, it is desirable that the abrasive layer contain a certain percentage of diamonds as, for example, from 5 to As the abrasive layer is comparatively mold thereto a backing is of metal powder containing no diamonds. It is also desirable to add a cheap abrasive or a filling material to the backing in order to minimize warping during curing. After the ring is sintered it can be mounted on any suitable backing 14, such as for example, a.

backing of a reversible thermoplastic resin. If

it is desired, the backing is can be made of metal and the ring soldered or otherwise attached thereto.

In making an article of high abrasive content from materials such as boron carbide, silicon carbide, fused alumina, the article can be preformed from a mixture of abrasive grains and a suitable binder, and then impregnated with silver or a silver alloy. This impregnation can be carried out by surrounding the article with solid metal,

and heating the two materials under vacuum or reduced pressure until the silver or the alloy melts and permeates the pores of the abrasive article. A material of this nature is practically non-porous and the silver formsa tough supporting matrix for the abrasive. If the abrasive particles are very finely divided, the material formed is sumciently homogeneous so that it can he used for bearings, tool tips and other similar purposes. Diamonds can be admixed with the principal abrasive if desired, or the diamonds themselves can be formed into a shaped article and impregnztted.

For the impregnation of abrasive bodies which are self-bonoled or which will withstand a very high temperature, the article can be surrounded with solid or liquid metal, and the temperature of the article rapidly raised until the metal flows into the pores of the article at atmospheric pressure. With recrystellizerl" or self-bonded silicon carbide, this temperature is about 19cc C. Silver hes a unique advantage for this purpose, in that it forms neither a silicicle nor a carbide, Whereas practically all other metals combine with either one or both of these elements, and when used to silicon carbide form either brittle silicicles or unstable carbides.

The abrasive wheels shown in the drawing are merely illustrative examples, and. other types of abrasive articles such as lens grinding laps, cup wheels for the surfacing of glass and refractory shapes, cut-oil wheels for glass, tungsten cerhicle and other hard substances, drills, insert teeth for stone saws, and wheels for petrographic thin sections, can be producecl' by the process herein described.

By the term silver base alloy as used in this specification and the appended claims is meant an alloy of silver and one or more other alloy ingredients, in which alloy silver is the predominant metal.

By the term eutectic alloy as used in this specification and the appended claims is meant the alloy solidifying at constant temperature having the lowest melting point of the system.

By the term solid solution" as nserl in this specification and the appended claims is meant an alloy constituent in which the crystals oi one metal in separating from the liquid contain apprecieble amounts of some other motel or compound. The crystals of such a constituent are homogeneous, are not themselves compounds, and cannot be resolved into riifierent constituents under the microscope.

The invention can be defined as being; within the scope of the following claims:

1. An abrasive article consisting of abrasive material comprising diamonds embedded and dispersed in a sinterecl metsl matrix consisting principally or" a oluctile silver-case ollcy which. alloy contains as a herdenins agent at least one metal other than silver retsinezl in solid. solution, in such an amomit to harden the matrix out not to destroy its cluctility.

2, An abrasive article consisting of material comprising alien-wilds emloerlriecl en persecl in a sintereo. metal matrix consist? clpslly of a ductile silver-loose alloy alloy contains copper as hardening agent retell-ice. 1 solid solution, such an amount to hereon the matrix but not to destroy its ductility.-

3. An abrasive article consisting of ehresive material comprising diamonds embedded and illspersed in aslnterezl metal matrix composer of a ductile alloy consisting principally of silver and containing, at least in part in solid solution, as hardening alloying materials, copper and a metal of the group consisting of zinc and cadmium, in such amounts as to harden the metre; but not to destroy its ductility.

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